question about prep work

What do you guys use for prep work? I read that if you have a good outer paintjob, and just repspray it that it will last okay. on the other hand I read athat it is good to sand everything down and spray it with primer.

Things seem to go both ways when reading up on this through articles. I would like to know what some of you guys who paint use as prep work. If you said it down what grit do you start out with, and what do you work your way up to? Do you primer the car?